1. Field of the Invention
This invention relates to a network interface system, in particular to those networks having respective the different speeds for the transfer of data.
2. Description of the Related Art
A token ring local area network (LAN) is a data communications system connected in a one-way signal path.
As to an access means of the data communications system, there are three common types of data transmission networks which are the CSMA/CD (EThernet) for IEEE 802.3, the token bus, and the token ring. Among these three types, only the token ring has two data transferred speeds, 4 Mbps and 16 Mbps (Mega Bits Per Second). Both of the transfer speeds are frequently used, and often the data transferred speed of 4 Mbps is used in one network while the data transfer speed of 16 Mbps is used in another network, both of which a user may wish to access.
FIG. 1 shows a conventional data communications system. In FIG. 1, a communication interface 1 is connected to of an analogue circuit. The interface 1 changes the network data transferred by network 4 into a digital signal as communication data. It concurrently generates a synchronizing clock signal 7 for transfer of the communication data. A network control unit 2 receives the communication data using the synchronizing clock signal 7 and the network control unit 2 transfers the communication data to the user unit, here shown as a data processing unit 3. On the other hand, information data generated by the data processing unit 3 is output to the network control unit 2. This information data is changed to an analogue output signal by the communication interface 1 in order to output it to the network 4.
FIG. 2 shows the details of the communication interface 1 shown in FIG. 1. The interface 1 includes a first module 21 for 4 Mbps data transfer and a second module 22 for 16 Mbps data transfer, which respectively have a switch 23, 24 for connecting to the network cable 4, an amplifier 25, 26 for amplifying the communication data transferred from the network 4 through the switch 23, 24, and a VCO (voltage controlled oscillator) controller 35, 36. Each VCO controller 35, 36 has a comparator 27, 28, a charge pump 29, 30 and low-pass filter 31, 32, for adjusting the phase difference between the synchronizing clock signal and the basic clock signal output of VCO 33, 34. The VCO 33, 34 generates the basic clock signal in response to the output of the VCO controller 35, 36. Both modules have a crystal oscillator located in the VCO 33, 34. The first module 21 for 4 Mbps data transfer has a 8 MHz crystal oscillator, and the second module 22 for 16 Mbps data transfer has a 32 MHz crystal oscillator. One of the modules must be selected by the user before the data communications system is connected to the network 4 so that the data communications system is synchronized with the frequency of the communication data transferred by the network 4. It is possible to select one module by means of making either switch 23 or switch 24 conductive in response to register 37 controlled by CPU 38. The clock signal generated by the selected module is also outputted to a data transmitter 39, a data receiver 40 and the network control unit 2 by way of a switch 29.
The communication interface of prior system has the following drawbacks.
First, the user needs to know the speed of the transferred data from the other network stations before the data communications system is connected to other network stations.
Second, the stations must have two modules for 4 Mbps and 16 Mbpsdata transfer respectively. As a result, much space is required. In a portable computer, for example for a Lap Top Computer, this is a serious problem which results in bigger and heavier unit.
Finally even if both modules are incorporated into an integrated circuit, there is no automatic selection of data transfer speed.